IPA: /ˈslɛndərnəs/
KK: /ˈslɛndərnəs/
The quality or state of being thin and graceful in form.
Her slenderness made her look elegant in the dress.
Slenderness is formed from "slender" (from Old English *slendrian*, meaning to be thin or lean) and the suffix "-ness" (meaning the state or quality of). The word "slenderness" refers to the quality or state of being slender or thin.
Think of the word 'slender' which means thin, and remember that 'slenderness' describes the quality of being thin.